flake.nix: Move vm out of nixosConfigurations so that checks work

lib/default.nix

@@ -1,4 +1,9 @@
-{lib}: let
+{
+  lib,
+  nixpkgs,
+  sops-nix,
+  tlaternet-webserver,
+}: let
   inherit (lib.attrsets) mapAttrsToList;
   inherit (lib.strings) concatStringsSep;
 in {
@@ -7,4 +12,19 @@ in {
     (mapAttrsToList
       (host: vm: "hostfwd=::${host}-:${vm}")
       portMapping);
+
+  makeNixosSystem = {
+    system,
+    extraModules,
+  }:
+    nixpkgs.lib.nixosSystem {
+      inherit system;
+      modules =
+        [
+          sops-nix.nixosModules.sops
+          tlaternet-webserver.nixosModules.default
+          (import ../configuration)
+        ]
+        ++ extraModules;
+    };
 }